Soluble Uridine Diphosphate d-Galactose:d-Glucose β-4-d-Galactosyltransferase from Bovine Milk

Abstract Some of the properties of a 70-fold purified bovine milk lactose synthetase have been determined. The enzyme preparation appears to be specific for uracil nucleoside diphosphate d-galactose derivatives. Adenosine, thymidine, cytidine, or guanosine diphosphate derivatives cannot serve as galactosyl donors for d-glucose to form lactose. The enzyme preparation will not transfer the galactosyl residue from uridine diphosphate d-galactose to α-d-glucose 1-phosphate, α-d-galactose 1-phosphate, d-xylose, maltose, α-methyl-d-glucoside, or l-glucose. N-Acetyl-d-glucosamine can serve as an acceptor for d-galactose but is only 25% as effective as d-glucose; the product of this reaction appears to be an analogue of lactose in which N-acetyl-d-glucosamine is substituted for d-glucose. It is not known whether these galactosyl transfer reactions are catalyzed by the same or a different enzyme. No reversal of the enzymatic reaction could be shown when lactose and UDP were used as substrates. The bovine milk lactose synthetase is activated by several divalent cations. The enzyme shows a maximum activation by Mn++, and an inhibition by ethylenediaminetetraacetate or Hg++. The enzyme has a 42° temperature optimum, and a pH optimum of 7.5, with maximum activity in sodium cacodylate-HCl or β,β'-dimethylglutarate-NaOH buffer. Phosphorylated compounds structurally related to the nucleoside diphosphate moiety of UDP-d-galactose strongly inhibit the enzymatic reaction whereas neutral hexoses or hexose phosphates produce little or no inhibition. The Km for UDP-d-galactose is 5.0 x 10-4 m, and that for d-glucose is 2.5 x 10-2 m.